Electrosynthesis of polyaniline–molybdate coating on steel and its corrosion protection performance

Electrosynthesis of polyaniline–molybdate (PANI–MoO₄²⁻) on mild steel was achieved in oxalic acid medium using cyclic voltammetry technique. Adherent and homogeneous PANI–MoO₄²⁻ coating was obtained. The corrosion behavior of steel with PANI–MoO₄²⁻ coatings in 1% NaCl solutions was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The coating was characterized by SEM, XPS, EDAX and FTIR. The self-healing ability of PANI–MoO₄²⁻ coating was confirmed by SVET technique. It has been found that the PANI–MoO₄²⁻ coating is able to offer higher corrosion protection in comparison to that of pure PANI coating due to inhibitive nature of molybdate ions.     

Influence of ionic speciation on electrocatalytic performance of polyaniline coated platinum electrode: Fe(III)/Fe(II) redox reaction

Porous-polyaniline coated Pt electrode (PANI/Pt) was electro-synthesized potentiodynamically in 0.1 M aniline + 0.5 M H₂SO₄ and morphologically characterized by scanning electron microscopy (SEM). Nature of predominant Fe-species in HCl and H₂SO₄ was checked by UV-vis spectrophotometry. Electrocatalysis of Fe(III)/Fe(II) reaction was studied by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for three different solution compositions viz. (i) FeCl₃/FeCl₂ in 1 M HCl, (ii) FeCl₃/FeCl₂ in 0.5 M H₂SO₄ and (iii) Fe₂(SO₄)₃/FeSO₄ in 0.5 M H₂SO₄. For different thicknesses of PANI, the peak current increased irrespective of the nature of the Fe-species, but the polarity of the charge on the Fe-species showed great influence on reversibility of electrocatalysis by PANI/Pt. The Donnan interaction of the polyaniline modified electrode for the three compositions was investigated with respect to [Fe(CN)₆]³⁻/H₂[Fe(CN)₆]²⁻ which are believed to be the predominant species present in K₃[Fe(CN)₆]/K₄[Fe(CN)₆] solution in 0.5 M H₂SO₄. The electrocatalytic performance of PANI/Pt for Fe(III)/Fe(II) redox reaction was found superior in HCl compared to that in H₂SO₄.     

Electrocatalytic reduction of bromate ion using a polyaniline-modified electrode: An efficient and green technology for the removal of BrO3 in aqueous solutions

The electrocatalytic reduction of bromate ion (BrO₃⁻) was investigated in a three-electrode system using polyaniline (PANI) as the electrode material. Bromate ion reduction and Br⁻ removal were observed during electrochemical treatment because of the catalytic and doping capabilities of the PANI film. BrO₃⁻ removal efficiency in the 0.10 mol L⁻¹ Na₂SO₄ supporting electrolyte achieved 99% at pH 7 in 25 min, with no bromide ion detected in the solution. Optimal removal was found in pH range 6-7, and the pH of the solution had a significant impact on bromate reduction. A reduction mechanism was also discussed by analyzing the cyclic voltammograms of the reduction process and X-ray photoelectron spectra of the main elements (N 1s and Br 3d) on the PANI surface. We propose that during the electrocatalytic reduction process, bromate is reduced to bromide because of the loss of electrons from the nitrogen atoms on the PANI chains. The doping of the resultant Br⁻ ions in the PANI film has an important role in avoiding further oxidation of Br⁻ to BrO₃⁻. The used PANI film can be regenerated by de-doping the Br⁻ ions with a 0.5 mol L⁻¹ H₂SO₄ solution. Thus the process can be considered efficient and green.     

Enhanced electrochemical stability and charge storage of MnO2/carbon nanotubes composite modified by polyaniline coating layer in acidic electrolytes

Manganese dioxide/multiwalled carbon nanotubes (MnO₂/MWCNTs) were synthesized by chemically depositing MnO₂ onto the surface of MWCNTs wrapped with poly(sodium-p-styrenesulfonate). Then, polyaniline (PANI) with good supercapacitive performance was further coated onto the MnO₂/MWCNTs composite to form PANI/MnO₂/MWCNTs organic-inorganic hybrid nanoarchitecture. Electrochemical performance of the hybrid in Na₂SO₄-H₂SO₄ mixed acidic electrolytes was evaluated by cyclic voltammetry (CV) and chronopotentiometry (CP) in detail. Comparative electrochemical tests revealed that the hybrid nanoarchitecture could operate in the acidic medium due to the protective modification of PANI coating layer onto the MnO₂/MWCNTs composite, and that its electrochemical behavior was greatly dependent upon the concentration of protons in the acidic electrolytes. Here, PANI not only served as a physical barrier to restrain the underlying MnO₂/MWCNTs composite from reductive-dissolution process so as to make the novel ternary hybrid material work in acidic medium to enhance the utilization of manganese oxide as much as possible, but also was another electroactive material for energy storage in the acidic mixed electrolytes. It was due to the existence of PNAI layer that an even larger specific capacitance (SC) of 384 F g⁻¹ and a much better SC retention of 79.9% over 1000 continuous charge/discharge cycles than those for the MnO₂/MWCNTs nanocomposite were delivered for the hybrid in the optimum 0.5 M Na₂SO₄-0.5 M H₂SO₄ mixed acidic electrolyte.     

Electropolymerization of aniline in acid media on the bare and chemically pre-treated aluminum electrodes: A comparative characterization of the polyaniline deposited electrodes

The electrosynthesis of polyaniline on the bare aluminum and pre-treated aluminum surface achieved in aqueous H₂PtCl₆ solution saturated with NaF for few seconds is described. The effect of some factors such as pre-treatment time, aniline and sulfuric acid concentrations on the electropolymerization process was investigated and optimum conditions were obtained. The stability of polyaniline film on the pre-treated aluminum electrode (Al-Pt) was studied as function of the potential imposed on the electrode. For applied electrode potentials of 0.1-0.7 V, the first-order degradation rate constant, k, of polyaniline film varies between 1 × 10⁻⁶ and 2 × 10⁻⁵ s⁻¹, and a relatively low slope (i.e. 2.1) was obtained for the plot of log k versus E. The coatings were characterized by scanning electron microscopy (SEM), and cyclic voltammetric behavior of the polyaniline-deposited Al electrode (Al/PANI) and polyaniline-deposited Al-Pt electrode (Al-Pt/PANI) in 0.1 H₂SO₄ solutions is described. The electrocatalytic activity of the Al-Pt/PANI electrode against para-benzoquinone/hydroquinone (Q/H₂Q) and Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ redox systems was investigated and the obtained results are compared with those obtained on Al/PANI and bulk Pt electrodes.     

Electroreduction of oxygen on gold-supported nanostructured palladium films in acid solutions

The electrochemical reduction of oxygen on thin Pd films with a nominal thickness of 0.25-10 nm on polycrystalline Au substrate (Pd/Au) was studied. The Pd films were prepared by electron beam evaporation and oxygen reduction was studied in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions using the rotating disk electrode (RDE) method. The surface morphology of Pd overlayers was examined by scanning tunnelling microscopy (STM). O₂ reduction predominantly proceeds through 4e⁻ pathway on all Pd/Au electrodes. The specific activity (SA) of oxygen reduction was lower in H₂SO₄ solution and decreased slightly with decreasing the Pd film thickness. In HClO₄, the SA was higher and not significantly dependent on the film thickness. The Tafel slope values close to −60 mV at low current densities and −120 mV at high current densities were found for all electrodes.     

Effect of water on inorganic acids doped polyaniline

Based on gravimetric measurements, the effect of water on thin films of inorganic acids doped polyaniline (PANI) was developed. The starting point was the fact that PANI coating on the electrode of quartz crystal microbalances (QCM) showed significant frequency shifts under exposure to liquid water. The changes in the frequency as a function of treatment time in water were quantitatively measured. These changes suggested that the mass decreases under water exposure were due to dopant ions release. The data have been collected using doped PANI films with HCl, H₂SO₄, and H₃PO₄. For PANI‐HCl upon immersion in water showed rapid mass loss followed by slow kinetically dopant ions release with time. However, PANI‐H₃PO₄ and PANI‐H₂SO₄ showed a slow kinetically release out of the film immediately upon immersion in water. The release process was well described by Fickian diffusion process. The diffusion coefficients (D) were determined and found to be dependent on the acid dopant utilized. They varied within the range of (1.68–14.7) × 10^‐14 cm²/s. This work presented an attempt to find a simple method based on the QCM for investigating the diffusion of dopant ions out of thin PANI films upon immersion in water and determining D. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Electrochemical and spectroscopic characterization of a tungsten electrode as a sensitive amperometric sensor of small inorganic ions

Cyclic voltammetry was used to investigate the electrochemical behaviour of the tungsten oxide films toward the electroreduction of BrO₃⁻, ClO₂⁻ and NO₂⁻ ions in acidic medium. The effects of the temperature, scan rate, pH, chemical composition of the electrolytic solutions, were investigated and the reduction mechanism was critically discussed.The reduction currents, evaluated in cyclic voltammetry and measured at −0.250 V versus SCE, increased linearly on increasing analyte concentration up to 20, 55 and 45 mM for nitrite, chlorite and bromate ions, respectively. The detection limits, evaluated in cyclic voltammetry, were 0.1, 0.4 and 0.7 mM for BrO₃⁻, ClO₂⁻ and NO₂⁻, respectively.The tungsten oxide film was successfully characterized as an amperometric sensor for the analytical determination of BrO₃⁻, ClO₂⁻ and NO₂⁻ ions in flowing stream. Operating under constant applied potential of −0.3 V versus Ag/AgCl the good reproducibility of the peak height and background current level during consecutive injections, indicates the absence of fouling effects and the potential applicability of the amperometric sensor for the routine analytical determination of the investigated inorganic ions. Considering the low values of the background currents (ca. 1.1 ± 0.1 μA) obtained in acidic and not deoxygenated carrier electrolyte, the tungsten sensing electrode seems to compete favourably with other common sensors for the amperometric determination of electroactive molecules under cathodic conditions.The X-ray photoelectron spectroscopy technique (XPS) was used in order to evaluate the chemical composition of the tungsten film upon electrochemical treatment in 0.1 M H₂SO₄ solution. Independently of the electrochemical treatment in acid solution, the tungsten surface electrode is generally composed by 50-60% of W⁰, 35-40% of W⁶⁺ and traces of W²⁺ oxide species.     

In situ corrosion monitoring of Ti–6Al–4V alloy in H2SO4/HCl mixed solution using electrochemical AFM

Electrochemical atomic force microscope (ECAFM) was employed for in situ observation of corrosion of solution annealed and furnace cooled Ti–6Al–4V titanium alloy in 0.5 mol l⁻¹ H₂SO₄ + 1 mol l⁻¹ HCl mixed solution. A scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) was also used for microstructure examination and chemical composition analysis. For solution annealed followed by furnace-cooled Ti–6Al–4V titanium alloy, selective corrosion of α phase and galvanic effect at α/β interface could be clearly observed at open circuit potential under ECAFM. A higher dissolution rate was also found in α phase than β phase on the bare Ti–6Al–4V titanium alloy. The effect of potential on the corrosion behavior was also explored. Negligible corrosion was found after potentiostatic etching at −0.5 and −0.85 V_Pt for 120 min. However, selective dissolution of α phase with respect to β phase occurred when the potential was controlled at −0.9 V_Pt. The polarity inversion during potentiostatic etching at −0.9 V_Pt was also found and discussed in this study.     

Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25-20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O₂ reduction was higher in HClO₄ and decreased with decreasing film thickness. In H₂SO₄, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec⁻¹ in high current density range and −60 mV dec⁻¹ in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O₂ reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O₂ molecule was close to four for all thin Pt films studied.     

Exchange of counter anions in electropolymerized polyaniline films

Mobile counter anion exchange of electropolymerized polyaniline (PANI) films with the anions in acid solutions has been investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy, Raman, UV-vis, and fluorescence spectra. The studied acids include camphorsulfonic acid, p-toluene sulfonic acid, HCl, HClO₄, H₂SO₄ and H₃PO₄. In two cases of small and large counter ions of PANI films, CV tests have been performed alternately three times in each of two electrolytes containing organic or inorganic acid. The investigation of electrochemical and spectrophotometric measurements reveals that large counter anions can be easily replaced by the small anions; and the reverse exchange also occurs, but shows very low efficiency. This indicates that the achieved anion exchange in both cases leads to the remarkable alteration in electrochemical behavior and electrical conductance of PANI films. This implies counter anions, whatever the original or redoping ones, play an important role in the electrical, electrochemical, even optical properties of PANI. However, the polymer morphology does not be changed after exchange tests. This allows us to point out a ‘morphology-memory’ effect which is very significant for the development and application of PANI films with the same morphology and different properties.     

A passivation mechanism of doped polyaniline on 410 stainless steel in deaerated H2SO4 solution

To investigate the role of polyaniline (PANI) in the corrosion protection of stainless steel (SS) in oxygen-deficient acidic solution, a separate doped PANI film electrode on a glass substrate was prepared and the test solution (1 M H₂SO₄) was purged with high-purity N₂ until dissolved oxygen level decreased more than two orders of magnitude. In this deaerated 1 M H₂SO₄ solution, the galvanic coupling interaction between the separate PANI film electrode and 410 SS was studied. Results reveal that the separate PANI film can passivate the 410 SS steadily for a long period of time. A variety of experimental methods including potentiodynamic measurement, potentiostatic (current-time) examination and X-ray photoelectron spectroscopy (XPS) are used to explore the mechanism by which the separate PANI film passivated the galvanic coupling SS in the deaerated sulfuric solution. These studies show that passivation is achieved because PANI film provides a large critical current at the early stage of coupling and a persistent passive current by its electrochemical dedoping/re-doping equilibrium activity with the acidic environment at the subsequent stage of coupling.     

Influence of Dopant Ions on the Properties of Conducting Polyacrylamide/Polyaniline Hydrogels

Polyaniline-impregnated polyacrylamide conducting hydrogels have been synthesized chemically via interfacial polymerization route using different acidic doping agents like sulfuric acid (H₂SO₄), hydrochloric acid (HCl), and perchloric acid (HClO₄). The best properties were found in case of H₂SO₄-doped polyacrylamide/polyaniline sample when compared with other doping agents. The resulting hydrogel exhibits superior properties including compact structure, high crystallinity, good mechanical strength, and electrical conductivity. The maximum electrical conductivity of the order of 9.4 × 10^?5 S/cm was found in case of H₂SO₄ doped polyacrylamide/polyaniline.     

High capacitance properties of polyaniline by electrochemical deposition on a porous carbon substrate

Electrochemical deposition of polyaniline (PANI) is carried out on a porous carbon substrate for supercapacitor studies. The effect of substrate is studied by comparing the results obtained using platinum, stainless steel and porous carbon substrates. PANI deposited at 100 mV s⁻¹ sweep rate by potentiodynamic technique on porous carbon substrate is found to possess superior capacitance properties. Experimental variables, namely, concentrations of aniline monomer and H₂SO₄ supporting electrolyte are varied and arrived at the optimum concentrations to obtain a maximum capacitance of PANI. Low concentrations of both aniline and H₂SO₄, which produce PANI at low rates, are desirable. The PANI deposits prepared under these conditions possess network morphology of nanofibrils. Capacitance values as high as 1600 F g⁻¹ are obtained and PANI coated carbon electrodes facilitate charge-discharge current densities as high as 45 mA cm⁻² (19.8 A g⁻¹). Electrodes are found to be fairly stable over a long cycle-life, although there is some capacitance loss during the initial stages of cycling.     

Electrodeposition of CdS from acidic aqueous thiosulfate solution—Invesitigation of the mechanism by electrochemical quartz microbalance technique

Electrochemical quartz crystal microbalance was used to study mechanism of cathodic electrodeposition of CdS from acidic aqueous solutions containing 0.01 M Cd(ClO₄)₂ and 0.1 M Na₂S₂O₃ as a source of sulfur. Experiments were performed by means of cyclic voltammetry and potentiostatic method. A comparison of gravimetric and current responses at pH 3 and 4 allowed for determination of the potential range in which side reactions of reduction of SO₃²⁻ and H⁺ ions compete most strongly with formation of CdS. The film thickness was determined by means of two methods: from AFM profiles and EQCM measurements.     

Electrocatalytic reduction of dioxygen at a modified glassy carbon electrode based on Nafion-dispersed single-walled carbon nanotubes and cobalt–porphyrin with palladium nanoparticles in acidic media

Cathodic dioxygen (O₂) reduction was performed at a modified glassy carbon electrode (GCE) by single-walled carbon nanotubes (SWCNT)/Nafion^® (NF) film with cobalt (II) tetra (2-amino-phenyl) porphyrin (CoTAPP) and palladium (Pd) nanoparticles incorporated and employed as doping agents. Both the electrochemical behavior of SWCNT with a P(CoTAPP)–Pd nanoparticle matrix and the electrocatalytic reduction of O₂ were investigated using transmission electron microscopy (TEM), cyclic voltammetry (CV) and rotating ring-disk electrode (RRDE) techniques in 0.1 mol l⁻¹ H₂SO₄ aqueous solutions. The electrocatalytic reduction of O₂ at the SWCNT/NF/P(CoTAPP)–Pd composite film established a pathway of four-electron transfer reductions into H₂O. Hydrodynamic voltammetry revealed that the modified electrode was catalyzed effectively by the four-electron transferred reduction of dioxygen into H₂O with minimal generation of H₂O₂. The SWCNT/NF/P(CoTAPP)–Pd composite film showed a highly efficient electrocatalytic performance. P(CoTAPP)–Pd was an effective mediator for the reduction of dioxygen and was responsible for the enhanced catalytic activity.     

The corrosion behavior of Fe-10Cr nanocrystalline coating

Fe-10Cr nanocrystalline (nc) coatings with a grain size of 20-30 nm were synthesized on glass substrates by magnetron sputtering. The corrosion behavior was investigated in 0.05 mol/L H₂SO₄ + 0.25 mol/L Na₂SO₄ and 0.05 mol/L H₂SO₄ + 0.5 mol/L NaCl solution by polarization curves, EIS and Mott-Schottky analysis. The results showed that compared to Fe-10Cr cast alloy, the active dissolution of the coating was accelerated; the passive film contained more Cr and therefore the coating was easier to passivate. The passive films formed on Fe-10Cr nc and cast alloy exhibited n-type semiconducting behavior in acidic solutions without Cl⁻ and p-type semiconducting behavior in acidic solutions with Cl⁻. The lower breakdown potential for both materials in the solution with Cl⁻ is related to the p-type passive film formed on them. For Fe-10Cr nc, lower donor density and increased Cr content were responsible for the chemical stability of the passive film.     

Nucleation and growth of anodic films on stainless steel alloys I. Influence of minor alloying elements and applied potential on passive film growth

The influence of minor alloying elements (Mo, V, W) when added to a Fe18Cr alloy on the ability of a passive film to nucleate and grow on a freshly generated metal surface, and on the subsequent stability of the film was investigated as a function of electrolyte composition and applied potential using a scratch chronoamperometric technique. Mo and V decreased the rate of active dissolution prior to passivation, allowing the onset of passivation to occur more rapidly, and also improved the stability of the passive film, especially to attack by Cl^– in acidic (H₂SO₄, HClO₄) solutions. W additions had a detrimental effect on the repassivation behaviour of Fe18Cr. Repassivation of the scratch scars was evaluated, from the current transients, in terms of the number of layers of surface film formed.     

Part-A: Synthesis of polyaniline and carboxylic acid functionalized SWCNT composites for electromagnetic interference shielding coatings

Polyaniline (PANI) was synthesized by chemical oxidation process by using Ammonium persulphate (APS) as an oxidizer and HCl as a dopant. The effects of altering the stoichiometric ratio of monomer to oxidizer, addition time, reaction temperature and dopant concentration on the electrical conductivity of PANI were studied in detail. The synthesis procedure was optimized to yield the PANI with maximum electrical conductivity. The pure PANI thus synthesized exhibited the maximum electrical conductivity 5.6 Scm⁻¹. Different PANI–cSWCNT composites were prepared by ex-situ and in-situ methods and a comparative evaluation of electrical conductivity was carried out. From the electrical conductivity measurements, it was seen that maximum conductivity 27.12 Scm⁻¹ was achieved for 20% cSWCNT loaded PANI composite prepared by in-situ method and 12 Scm⁻¹ for the same composite prepared by ex-situ method. The efficacy of in-situ method, for conductivity enhancement was attributed to the formation of PANI coating over cSWCNT during the synthesis. This coating formation was further substantiated by FTIR, XRD, DSC, TGA, FESEM, and HRTEM analysis. The results of these studies confirmed that the in-situ prepared 20% cSWCNT loaded PANI composite is the most preferred filler for developing polyurethane based EMI shielding coatings.     

The electrochemical synthesis of polyaniline/polysulfone composite films and electrocatalytic activity for ascorbic acid oxidation

Polyaniline (PANI)/polysulfone (PSF) composite films with asymmetric porous structure were successfully prepared by electropolymerization. The back face (in contact with the electrode) of the freestanding composite film is green while the outer face is white. The chemical component and the morphology of the surfaces were characterized by FTIR spectra and scanning electron microscopy, respectively. It was shown that replicate films gave reproducible voltammetry in 0.5 M H₂SO₄. The influence of the electrolyte and the acidic concentration on the redox peak currents of polyaniline were investigated in detail. The composite film electrode showed good electrocatalytic activity for ascorbic acid, which the anodic overpotential was evidently reduced compared with that obtained at bare Pt electrode. The diffusion coefficient of ascorbic acid was 1.38 × 10⁻⁶ cm² s⁻¹.